Yes but whatever you fill the memory with still has to be processed and rendered with the CPU + GPU.
Cerny said that streaming was a main reason for the 5.5gb SSD and he said what if you can load textures in that 1/2 second, he then goes on to say thats 4gb of compressed data you can load, which would Free up RAM space.
But ultimately somthing is going to bottleneck first, if you can have 4gb of textures in a scene stream from the SSD and fill the ram with, geometry, lighting calculations, sound, Ai etc then with your going to reach a limit with the memory you have, what the GPU can render or what the CPU can draw.
Judging by what we have seen so far its not hard to fully utilise bthe GPU.
Some examples from the platforms of there approaches would of been nice, the best one Ive seen is the SFS demo from Microsoft, even though it has simplistic geometry and lighting it does demonstrate how the size of textures can be greatly reduced.